CN1322518C - Conductive paste for terminal electrode of multilayer ceramic electronic part - Google Patents

Abstract

A conductive paste for terminal electrodes of multilayer ceramic electronic parts, comprising (A) a spherical conductive powder mainly containing copper and having a glassy thin film on at least a part of its surface, (B) a flaky conductive powder mainly containing copper , (C) glass powder, and (D) organic vehicle, and may further include (E) aliphatic amine. When fired to form terminal electrodes of multilayer ceramic electronic parts, the paste exhibits very excellent low-temperature binder removal performance, and is also excellent in oxidation resistance, binder removal performance and firing performance, while not Firing conditions need to be strictly controlled, so that dense terminal electrodes excellent in both adhesion and conductivity can be formed.

Description

Conductive Paste for Terminal Electrodes of Multilayer Ceramic Electronic Parts

Background of the invention

1. Field of invention

The present invention relates to an electroconductive paste for forming terminal electrodes of multilayer ceramic electronic parts such as multilayer ceramic capacitors, multilayer ceramic inductors, multilayer piezoelectric elements and the like. In particular, the present invention relates to a conductive copper paste suitable for forming terminal electrodes of multilayer ceramic electronic parts having base metal internal electrodes made of nickel or the like.

2. Background technology

Multilayer ceramic electronic parts, for example, multilayer ceramic capacitors, are generally produced as follows. The internal electrode conductive paste is printed in a prescribed pattern on a dielectric ceramic green sheet such as barium titanate ceramic or the like. Several such individual layers are laminated and pressed together to prepare a green laminate in which ceramic green sheet layers and internal electrode paste layers are alternately laminated. The laminate thus obtained is cut into small pieces of a prescribed shape, and then these small pieces are co-fired at high temperature, thereby producing a multilayer ceramic capacitor main body. Secondly, by dipping or similarly coating the end face of each ceramic capacitor body where the internal electrodes are exposed with a terminal electrode conductive paste mainly composed of conductive powder, glass powder and organic vehicle; then, after drying, the terminal is formed by high-temperature firing. electrode. Subsequently, nickel, tin or the like plating is formed on top of the terminal electrodes by plating or the like if necessary.

Generally, noble metals such as palladium, silver-palladium, platinum, and the like have been used as internal electrode materials. However, base metals such as nickel, cobalt, copper, and similar base metals now constitute the mainstream of this material because of the need for resource conservation, cost reduction, and prevention of delamination and cracking due to oxidative expansion during sintering of palladium or silver-palladium. . Therefore, copper, nickel, cobalt, or alloys of these metals, which provide easy formation of an excellent electrical connection with base metal internal electrodes, are used instead of silver or silver-palladium as the terminal electrode material.

Where base metals are thus used for internal and terminal electrodes, the firing of the terminal electrodes is usually at a maximum temperature of 700 to 900°C in a non-oxidizing atmosphere with a very low partial pressure of oxygen containing, for example, several ppm to several tens of ppm under an inert gas atmosphere of oxygen.

However, when the terminal electrode conductive paste mainly composed of copper is fired in such an atmosphere containing a small amount of oxygen, organic components such as binder resins, solvents or the like used as a carrier do not tend to be oxidized. Therefore, proper combustion, decomposition and removal of organic components (also called "binder removal") is difficult to do in a proper manner. In particular, at the initial stage of firing at a relatively low temperature, if the binder is not sufficiently removed before glass fluidization and copper powder sintering, carbon and carbon-containing organic residues such as supports decompose after sintering begins The product or the like will be enclosed in a film. This occluded carbon and carbon-containing organic residues (hereinafter referred to as "residual carbon" in some cases) can cause various problems in subsequent high temperature stages, causing loss of performance of the electronic part, and reducing the reliability of the part. For example, at the stage of sintering copper powder at high temperature, the carbon remaining in the film hinders the fluidization of the glass and the sintering of copper, so that the excellent dense structure of the electrode and the adhesion to the ceramic body are damaged. Moreover, this residual carbon deprives oxygen in the dielectric ceramics, thus causing oxygen deficiency to cause a decrease in dielectric properties, and to cause a decrease in the strength of the ceramic body as well. As a result of the decrease in the strength of the ceramic body, cracks in the ceramic body due to thermal shock (thermal cracking) occur during a subsequent welding process or the like. Also, when the trapped residual carbon turns into gas at high temperature, bubbles (bubbles) are formed so as to affect the structure of the sintered film. As a result, when the plating treatment is performed on the sintered film later, the plating solution penetrates into the electrode film, causing a decrease in insulation resistance and cracks in the ceramic body; moreover, the infiltrated plating solution is heated and turned into a gas during solder reflow , thus resulting in "solder spatter", which causes the molten solder to scatter.

Therefore, the problem of how to effectively remove the binder in the initial stage of firing, so that the residual carbon is reduced before the copper powder is sintered in the high temperature zone, has become a problem in the terminal electrode conductive paste mainly composed of base metals (especially copper). an important question.

Generally, in order to solve this problem, a method of using a resin having good thermal decomposition properties such as an acrylic resin or the like as a binder resin, or using a resin having a property that the glass does not tend to soften at low temperatures but removes the carrier After softening the properties of the glass, the structure of the prepared electrode is better.

Also, terminal paste using fine spherical copper powder forms an overly dense film when applied and dried; therefore, it appears that the carrier is not easily removed, and carbon remains at high temperatures. Therefore, it has been proposed to use flaky copper powder instead of spherical copper powder. For example, Japanese Patent Laid-Open No. 8-180731A discloses a multilayer ceramic capacitor terminal paste comprising flaky copper powder, spherical copper powder, glass powder and an organic vehicle. This flaky copper powder formed suitable voids in the dried paste film; these voids were shown to act as gas channels so that binder removal proceeded smoothly in the structure. Also, in Japanese Patent Laid-Open No. 2002-56717A, it is pointed out that binder removal performance can be improved by setting paste dry film density within a specific range without sacrificing application performance and film fine structure of the paste.

Meanwhile, in order to effectively remove the binder, there are also methods in which, before densification of the electrode during temperature rise at the time of firing, by increasing the oxygen concentration to hundreds of ppm or higher, thereby accelerating the oxidative decomposition of organic materials, and then reducing the oxygen concentration and firing. For example, Japanese Patent Laid-Open No. 10-330802A and Japanese Patent Laid-Open No. 2001-338831A disclose methods in which small spherical fine copper powder is coated with an anti-oxidation coating composed of glass or the like, The binder removal process is performed in a pressurized atmosphere such as an air atmosphere or the like so that the support is decomposed while preventing oxidation of copper, and the firing is performed after reducing the partial pressure of oxygen.

In recent years, increased capacity, improved performance and improved reliability of multilayer ceramic electrode components have been increasingly demanded. Especially in the case of small-sized large-capacity multilayer ceramic capacitors, the spacing between internal electrodes becomes narrow, ie, 1 to 2 µm, so that defective capacitors are liable to occur if terminal electrodes are not dense and structured. There is therefore a need for smoother binder removal and formation of a denser and finer textured final fired film free of oxidation. However, in the case where the terminal electrode conductive paste used is a copper conductive paste, the reduction of the amount of residual carbon (excellent binder removal performance) and the prevention of oxidation of copper are conflicting purposes, that is, if one of the properties is tried to be improved, The other of these properties will be degraded, and if any one of these properties is poor, no good electrode will be formed. In addition, the influence of residual carbon in sintering copper is also so great that it is extremely difficult to satisfy the above requirements no matter which conventional method is used.

For example, after coating the surface of copper powder with a glass coating, removing the binder in an atmosphere of high oxygen partial pressure as described above, and performing firing after reducing the oxygen partial pressure, in the case of this method, The anti-oxidation effect and binder removal performance in the low temperature region are excellent; however, the adjustment of the atmosphere in the high temperature region is difficult, so that it is difficult to perform firing without eventually oxidizing the copper powder.

Conversely, however, when the binder is removed in a low-oxygen atmosphere having an oxygen partial pressure of several tens of ppm or less, the binder removal at low temperatures tends to be incomplete even by using flaky copper powder It has a structure in which gas can be easily discharged to form a thin film. This tendency is particularly remarkable when the oxygen concentration in the firing atmosphere is several ppm or less, or when the number of chips fired simultaneously is large. Also, in order to improve dispersibility and prevent oxidation, flake metal powders are usually surface-treated with fatty acids or metal salts thereof such as stearic acid or the like; however, according to studies conducted by the present inventors, the presence of such substances promotes foaming and degradation of the ceramic body.

Moreover, in the case of firing in a low-oxygen atmosphere, it is extremely difficult to strictly control the oxygen partial pressure to the order of about ppm and keep the atmosphere at a constant concentration. In particular, when the organic material contained in the paste decomposes, oxygen is taken from the atmosphere so as to cause a decrease in oxygen, and a decrease in metal oxidation occurs, so that a slight change occurs in the oxygen concentration. Therefore, the ease of binder removal and the degree of copper oxidation also vary slightly with the ceramic body, die size, number of dies fired simultaneously, organic components in the paste, and firing conditions such as oxygen concentration, maximum temperature, and temperature curves and similar conditions vary. Moreover, because the amount of carbon remaining in the firing process up to the high temperature region varies greatly with the number of simultaneously fired small pieces and the shape of these small pieces, to obtain a stable localized oxygen in the vicinity of the fired body Partial pressure is especially difficult. As a result, performance fluctuates so much that there is considerable scatter in the results.

However, there are currently differences in the types of ceramics and firing conditions used according to electronic parts standards and manufacturers, so that there is a need for a paste with a wide process way to obtain terminal electrodes with excellent performance.

Summary of the invention

The purpose of the present invention is to solve all the above-mentioned problems, and to provide a terminal electrode copper conductive paste, [i] when baked onto multilayer ceramic electronic parts, especially when having a partial pressure of oxygen of several tens of ppm or more When fired through a binder removal process and a high-temperature firing process in a low-oxygen atmosphere, it exhibits minimal copper oxidation and at the same time exhibits excellent binder removal properties at low temperatures,[ii] it does not Any blistering or degradation of the ceramic body would be caused, and [iii] it would form a dense, highly conductive sintered film without plating solution penetration or imperfect connection to the internal electrodes.

In particular, in addition, it is an object of the present invention to provide a copper conductive paste which has little sensitivity to the sintering temperature or the oxygen concentration in the firing atmosphere, and which can be applied to many different types of ceramic bodies and Ability to meet changes in firing conditions.

In order to achieve the above objects of the present invention, the present invention includes the following structures.

(1) A conductive paste for terminal electrodes of multilayer ceramic electronic parts, comprising (A) a spherical conductive powder mainly containing copper and having a glassy thin film on at least a part of its surface, (B) a flaky conductive powder mainly containing copper , (C) glass powder, and (D) organic vehicle.

(2) A conductive paste for terminal electrodes of multilayer ceramic electronic parts, comprising (A) a spherical conductive powder mainly containing copper and having a glassy thin film on at least a part of its surface, (B) a flaky conductive powder mainly containing copper , (C) glass frit, (D) organic vehicle, and (E) aliphatic amine.

(3) The conductive paste according to (2) above, wherein at least a part of the above-mentioned (E) aliphatic amine is adsorbed on the surface of the (B) flaky conductive powder particles.

(4) The conductive paste according to (2) or (3) above, wherein the amount of (E) aliphatic amine is 0.05 to 2.0% by weight based on the weight of (B) flaky conductive powder.

(5) The conductive paste according to any one of (1)-(4) above, wherein the ratio of (A) and (B) is in the range of 5:95 to 95:5.

The terminal electrode conductive paste of the present invention is remarkably excellent in oxidation resistance, binder removal performance and firing performance, and can form a dense terminal electrode with excellent adhesive strength and conductivity.

Therefore, even in the case of firing in an inert atmosphere with a low oxygen partial pressure, there is no decrease in electrical properties of the ceramic body due to residual carbon or the like, and no thermal cracking due to decrease in mechanical strength occurs. Furthermore, a highly reliable multilayer ceramic electronic part exhibiting excellent performance after a high-temperature load life test can be produced. Furthermore, a dense electrode film without air bubbles can be formed, so that there is no permeation of the electrolytic solution during plating following firing, and thus no cracks and lowering of insulation resistance, and no solder spatter. Moreover, there is also no increase in terminal electrode resistance due to oxidation of copper, no insufficient capacity due to imperfect connection with internal electrodes, and no decrease in plateability or the like.

Moreover, as a result of using a specific conductive powder and a specific dispersant, the paste of the present invention has little sensitivity to firing conditions, and can satisfy various firing conditions such as firing atmosphere (especially oxygen concentration), firing Temperature and profiles and similar conditions. Furthermore, the present invention can be applied to various ceramic bodies having different compositions and properties. Also, since strict control of firing conditions is not necessary, the method can be simplified, the production efficiency can be improved, and the cost can be reduced.

Detailed description of the preferred embodiment

The spherical conductive powder (A) mainly comprising copper and having a thin glassy film on at least a part of the surface of the powder used in the present invention is formed by coating a spherical metal powder mainly comprising copper with a thin glassy substance (or The thin glassy substance binds to the powder). In addition to pure copper powder, alloys mainly composed of copper, such as alloys containing copper and at least one metal selected from gold, silver, palladium, platinum, nickel, rhodium, cobalt, iron and the like, can also be used as Spherical metal powder consisting mainly of copper. (Hereafter, these powders are collectively referred to as "spherical copper powder"). A powder having a powder average particle diameter D ₅₀ (including a glassy thin film) of about 0.1 to 10 µm as measured by a laser diffraction particle size analyzer can be used as the spherical copper powder.

When the glassy thin film exists as a solid phase on the surface of the copper powder, the film functions as a protective layer against metal oxidation and as a layer against sintering. The individual particles that make up the spherical copper powder need not be completely coated with glassy material. However, it is thus desirable that the surface is uniformly coated with powder. The amount of the glassy thin film is preferably 0.01 to 50% by weight, and even more preferably 0.1 to 10% by weight, based on the weight of the spherical copper powder.

This powder is prepared by any desired method, such as a method of obtaining a glassy thin film by vapor deposition on spherical copper powder, or a method of coating the powder by a sol-gel method or the like. However, in order to form a very thin glassy film of uniform thickness covering the entire surface of a single copper particle, it is required to prepare the powder by using the method described in Japanese Patent Laid-Open No. 10-330802A, for example, the method is: solution forming micro droplets, the solution comprising at least one thermally decomposable copper compound and an oxide precursor that undergoes thermal decomposition to form a glassy material that does not form a solid solution with the above-mentioned metals, and by heating these micro-droplets to a ratio of the above-mentioned metal At a temperature at which the decomposition temperature of the compound is high, the glassy material is deposited in the vicinity of the surface of the copper metal powder at the same time as the copper powder is produced.

A glassy film is any glassy material having a glass transition point and a glass softening point at which the glass softens and liquefies during firing in the high temperature region after binder removal; the glassy film can Is amorphous, or can contain crystals in an amorphous film. In addition, the composition of this glassy thin layer may be the same as or different from that of the glass frit mixed with the paste as an inorganic binder; It does not soften or liquefy at its decomposition temperature, and softens and liquefies after binder removal so that the glass acts as a sintering aid. Examples of glass types used include BaO-ZnO-B ₂ O ₃ type, BaO-ZnO type, BaO-SiO ₂ type, BaO-ZnO-SiO ₂ type, BaO-B ₂ O ₃ -SiO ₂ type, ZnO- B ₂ O ₃ types, BaO-CaO-Al ₂ O ₃ types, PbO-B ₂ O ₃ -SiO ₂ -Al ₂ O ₃ types, PbO-B ₂ O ₃ -ZnO types, ZnO-B ₂ O ₃ types, Bi ₂ O ₃ -B ₂ O ₃ -SiO ₂ types and R' ₂ OB ₂ O ₃ -SiO ₂ types (R' represents an alkali metal element) and the like. If a glassy component with a high softening point is selected, oxidation and sintering will be inhibited before firing in the high temperature zone following binder removal; therefore the amount and composition of the coating depends on the firing conditions. Also, ingredients for improving the adhesion, plateability, conductivity and the like of the electrode film by reacting with the molten glass frit (C) during firing may also be included in the composition.

As in the case of the above-mentioned spherical conductive powder (A), besides pure copper powder, an alloy containing copper as a main component can also be used as the flaky conductive powder (B) mainly containing copper as specified in the present invention. Also, a thin film of an oxidation-resistant metal such as nickel, cobalt, iron, zinc, tin, gold, silver, palladium, platinum, rhodium, or the like, or an alloy of these metals, is deposited on a thin sheet by, for example, electroplating, vapor deposition, or the like. Surface formation of spherical copper powder particles, wherein a powder capable of forming a glassy thin film similar to the spherical conductive powder (A) can also be used. As a result of the formation of such metallic coatings or glassy films, oxidation resistance is improved so that firing at higher oxygen partial pressures is possible. Hereinafter, these powders will be collectively referred to as "flaky copper powder".

It is desirable to use a powder having an average particle diameter of 1.0 to 10.0 μm as the above-mentioned flaky copper powder. Herein, the average particle diameter is an average value of major axes of flaky particles, and is a cumulative fractional 50% value (D ₅₀ ) (weight basis) of particle size distribution measured by using a laser scattering particle size analyzer. By setting the average particle diameter within the above range, it is possible to form a dry film of conductive paste having a structure that allows decomposition products of the carrier (transformed into gas at the time of firing) to be easily discharged to the outside of the film; in addition, it is possible to obtain Excellent coating film shape. If the average particle size is less than 1.0 μm, the binder removal is insufficient, and air bubbles tend to occur; moreover, the oxidation resistance decreases. Also, if the average particle size exceeds 10.0 μm, the fluidity of the paste decreases, and it is impossible to apply Excellent shape; moreover, the porous structure formed during the drying of the film remains "porous" in the fired film so that the electrode tends to become porous.

In addition, especially, if the ratio of the average particle diameter (μm) to the average thickness (μm) of the flaky copper powder is set in the range of 3 to 80, or if the specific surface area is set in the range of 0.3 to 2.0 m ² /g Within the range, a terminal electrode conductive paste with a very good binder removal effect combined with excellent coating compatibility and firing performance can be obtained. If the ratio of the average particle diameter to the average thickness is less than 3, the binder removal performance is insufficient; in addition, if the ratio exceeds 80, the fluidity of the paste is reduced so that when coating is performed by dipping (such as forming protrusions, etc.) It is difficult to form a coating with an excellent shape. Additionally, electrodes tend to be porous, and the surface exhibits a tendency to be rough. However, the average thickness of the flaky powder can be determined by SEM observation. If the specific surface area is less than 0.3m ² /g, the electrode film obtained by firing tends to be porous; in addition, if the surface area is greater than 2.0m ² /g, the fluidity of the paste is insufficient, and the center portion of the terminal electrode tends to form a convex out.

Such flaky copper powder can be prepared by any method. For example, a method of grinding spherical powder using a ball mill or the like, a chemical reduction method, and a method of crushing copper foil can be used.

The mixing ratio of the above-mentioned spherical conductive powder and the above-mentioned flaky conductive powder can be appropriately determined according to the materials used, firing conditions, and required properties. However, preferably, the ratio is in the range of 5:95 to 95:5 (weight ratio). If the ratio of the flaky conductive powder is less than this value, the density of the dry film becomes too high so that the binder removal performance is lowered. On the other hand, if it exceeds this range, it is conversely difficult to discharge the carrier decomposition gas from the electrode film, so that the binder removal performance is lowered; in addition, the diffusion of metal is insufficient at high temperature, so that sintering is prevented.

Preferably, (A) and (B) are properly selected and mixed so that the dry film density D (g/cm ³ ) calculated by the following formula is in the range of 3.0 to 4.8 g/cm ³ .

D=W/(πT×10 ^-4 )

Here, W and T are the weight (g) and thickness (μm) of the dry film after the conductive paste is applied as a surface coating of the PET film so that the resulting film has a thickness of about 250 μm, dried at 150°C for 10 minutes and Then cut into discs with a diameter of not more than 20 mm, and remove the PET film.

There is no particular limitation on the glass powder (C) used, as long as the powder can be used as an inorganic binder in the common terminal electrode copper paste. In particular, anti-reduction glasses ideally used do not contain easily reducible components such as lead, etc., for example, BaO-ZnO-B ₂ O ₃ type, RO-ZnO-B ₂ O ₃ -MnO ₂ type, RO-ZnO type, RO - ZnO-MnO ₂ type, RO-ZnO-SiO ₂ type, ZnO-B ₂ O ₃ type, SiO ₂ -B ₂ O ₃ -R' ₂ O type glass and similar (R for alkaline earth metal and R' for alkali metal element). The amount of the glass powder mixed is about 1 to 20 parts by weight per 100 parts by weight of the conductive powder. In the case where the amount used is less than 1 part by weight, the bonding strength between the ceramic body and the terminal electrodes of the above-mentioned thin electronic part decreases. In addition, in the case where the amount exceeds 20 parts by weight, a large amount of glass is distributed on the electrode surface after firing, so that dissolution between fragments occurs, and it becomes difficult to plate on the surface of the terminal electrode.

In addition to the usual methods of mixing, melting, rapidly cooling and grinding raw material mixtures of the corresponding components, glass powders can also be obtained by other ideal methods such as sol-gel methods, spray pyrolysis methods, atomization methods or similar methods ． Especially in the case of the spray pyrolysis method, spherical glass frit having a small uniform particle size can be obtained so that pulverization treatment is not required when it is used for a conductive paste. Therefore, such an approach is ideal.

The conductive paste of the present invention may also contain various inorganic additives such as commonly used metal oxides, clay minerals, ceramics, oxidizing agents and the like, and other conductive powders in amounts that do not cause loss of the effects of the present invention.

There is also no particular limitation on the organic vehicle (D); a general-purpose organic binder dissolved and dispersed in an organic solvent such as an acrylic type resin, a cellulose type resin or the like can be appropriately selected and used. Plasticizers, dispersants, viscosity modifiers, surfactants, oxidizing agents, organometallic compounds and the like may also be added, if necessary. There is also no limitation on the mixing ratio of the carrier; the amount used is an appropriate amount for maintaining the inorganic components in the paste, and the amount can be appropriately adjusted according to the application method.

Various amines can be used as aliphatic amines (B), for example, primary amines such as octylamine, laurylamine, tetradecylamine, octadecylamine, oleylamine, tallowamine, tallow propylene diamine and the like, secondary Amines such as distearylamine and the like, tertiary amines such as triethylamine, dimethyloctylamine, dimethyltetradecylamine, dimethylpalmitamine, dimethylstearylamine, dimethylbehenylamine, Dimethyllaurylamine, trioctylamine and similar. Two or more of these amines may be used in combination. Mixtures of a number of different types of amines, often sold as "aliphatic amines," may also be used. In particular, higher alkylamines having about 14 to 18 carbon atoms in the main chain, such as deca Octadecylamine stearylamine, oleylamine, dimethylstearylamine or the like, or aliphatic amines comprising mainly such amines, are especially suitable for use.

The aliphatic amine is preferably used by being applied as a coating treatment and previously adsorbed on the surface of the flaky copper powder. There is no particular limitation on the coating method used; for example, amine may be used alone or dissolved in a solvent, and the surface of the copper powder is surface-treated by the same method used for common surface treatment agents. Also, in the case of preparing flaky copper powder by grinding spherical or granular copper powder, the aliphatic amine can be used as a grinding aid, and mixed with the paste when adsorbed on the surface of the powder "as an aid".

Aliphatic amines prevent oxidation of flake copper powder and improve powder dispersion in paste. Generally, higher fatty acids such as stearic acid, lauric acid and the like, or metal salts of such acids, are commonly used for this purpose. However, the fatty acid contained in the copper paste is so resistant to decomposition and dispersion under ordinary reducing firing conditions that the fatty acid is preserved up to a high temperature of 700 to 800° C. even after the carrier component has been completely decomposed. According to the studies carried out by the inventors, it is apparently due to the following reason: that is, as a result of the formation of metal soaps with copper, higher fatty acids are strongly bound to the surface of copper powder, so that in an atmosphere containing very little oxygen these The substance decomposes easily at high temperature. Therefore, even though support removal is improved, the fatty acid remains in the film as residual carbon, and begins to decompose only after the film begins to sinter. Therefore, this hinders sintering, and causes bubbles and degradation of the ceramic body, and increases process dependence. On the other hand, aliphatic amines have excellent antioxidant effects and functions as dispersants, and do not form compounds that strongly bind copper, so that these compounds are easily decomposed and removed from the electrode film even in a low-temperature non-oxidizing atmosphere. remove.

The amount of the aliphatic amine mixed into the components is preferably 0.05 to 2.0% by weight based on the weight of the flake copper powder. If the mixing amount is less than 0.05% by weight, the effect will be insufficient; in addition. Even if it is blended in an amount exceeding 2.0% by weight, there is no additional improvement.

In the paste of the present invention, as a result of mixing the above ingredients, it is conjectured that the structure of the dry film can be maintained in a porous state, which is optimal for the removal of the binder, so that the removal of the binder can be smoothly carried out in the structural unit conduct. Thus, binder removal can be performed very quickly without oxidation of the copper prior to softening the glass and sintering the copper, so that the amount of residual carbon remaining closed in the film until the high temperature region is minimized.

Second, sintering proceeds gradually, and the structure becomes dense and fine. According to a study conducted by the present inventors, during firing, for example, the oxygen partial pressure at which oxidation of copper does not occur at a peak temperature of 800°C is 10 ^-3 ppm or less. At this point, in order to maintain such a low oxygen partial pressure at this temperature, the amount of residual carbon is not zero; apparently it is necessary to retain a very small amount of carbon near the copper powder, so that an oxygen-depleting environment is formed. However, in the past, it has been very difficult to retain such small and controlled amounts of carbon regardless of whether the binder removal process was performed in an oxidizing atmosphere or in a very oxygen-poor atmosphere. In addition, in the examples described below, according to the findings confirmed by the present inventors, in the case of the inventive paste shown in the examples, residual carbon exists in a very small controllable amount near the copper powder in the high temperature region. As a result of the presence of carbon, apparently, even at elevated temperatures close to the maximum firing temperature, a partial pressure of oxygen is maintained locally in the vicinity of the copper powder so that oxidation of the copper is prevented.

Also, because of this ideal sintering behavior, and because of the controlled atmosphere that can be created, it is apparent that the sensitivity to firing conditions is also reduced. In particular, the amount of carbon uniformly preserved to the high temperature region is stable so that process dependence is small even in the case of mixing spherical copper powder and flaky copper powder so that the filling performance of the dry film is high.

Example

Preparation of samples 1 to 9

Mixing the corresponding components as shown in Table 1, using a uniform BaO-SiO _type glassy thin layer with an average particle size of 2 μm and an average thickness of 13 nm on the surface of the powder particles (glassy thin layer in the powder: about 2% by weight ) as a glass-coated spherical copper powder having an average particle diameter of 7 µm and an average Thick flaky copper powder As flaky copper powder (the amount of octadecylamine relative to the amount of flaky copper powder: about 0.3% by weight), a BaO-ZnO-B ₂ O ₃ type spherical glass powder with an average particle size of 2 μm is used Or ZnO-B ₂ O ₃ -SiO ₂ type spherical glass powder with an average particle size of 2 μm as glass powder, and a solution obtained by dissolving acrylic resin in terpineol as an organic vehicle, and then prepared conductive by kneading with a three-roll mill paste. Samples 6 to 9 are outside the scope of the present invention. Furthermore, in Samples 8 and 9, spherical copper powder having an average particle diameter of 2 μm without a glassy thin layer was used instead of the spherical copper powder coated with a glassy thin layer.

Experiment 1

A Y5V 1μF (rated value) multilayer ceramic capacitor body with a planar size of 2.0mm×1.25mm and a thickness of 1.25mm was prepared by sintering a stack of barium titanate ceramic dielectric green sheets and nickel internal electrodes at high temperature. Apply the conductive paste in samples 1 to 9 by immersing the exposed end faces of the nickel inner electrodes of the capacitor body so that the layer thickness after firing is 60 μm, and hold it in a hot air dryer at 150° C. for 10 minutes to dry the samples.

Thereafter, in the belt muffle furnace, set the whole area of the firing atmosphere (binder removal zone and firing zone) to contain 5ppm oxygen nitrogen atmosphere, and at the peak temperature shown in Table 1 and at the peak temperature The sample was fired with the holding time set at 10 minutes, and the time from the start to finish of firing was set at 1 hour, thus forming terminal electrodes and obtaining a multilayer ceramic capacitor.

The electrostatic capacity was measured with a capacitor having terminal electrodes baked under the corresponding conditions. Also, the surface and cross section of the terminal electrode film were observed with a scanning electron microscope (SEM), and the presence or absence of pores was checked.

Furthermore, for test samples in which a nickel plating layer was formed on an electrode film by electroplating and a tin plating film was further formed, plateability, terminal electrode tensile strength, and the presence or absence of solder spatter were examined. Furthermore, a thermal shock test was performed. The results are summarized in Table 1. In addition, the electrostatic capacitance and tensile strength values shown are average values of 500 capacitors.

Platability was estimated by the extent (%) of the plated area relative to the electrode surface. ○: substantially 10%, ○: 90 to 99%, Δ: 70 to 89% adhesion, ×: 69% or less.

The inspection with and without solder spatter, and the thermal shock test were performed as follows.

Amount of solder spatter: 30 samples in which terminal electrodes were covered with solder, the samples were passed through a solder remelting furnace, and the number of samples in which a phenomenon in which molten solder spattered to surrounding areas occurred was investigated.

Thermal Shock Test: Samples were immersed in a dry solder bath at 330°C for 7 seconds and the number of samples (out of 30 samples) showing thermal cracking was investigated.

Table 1

Sample No. 1 2 3 4 5 6* 7* 8* 9* Spherical copper powder coated with glassy thin film (parts by weight) 50 50 90 30 10 0 100 - - Spherical copper powder without glassy state film (parts by weight) - - - - - - - 30 50 Flake copper powder (weight powder) 50 50 10 70 90 100 0 70 50 BaO-ZnO-B ₂ O ₃ type glass powder (parts by weight) 6 - 4 8 8 12 4 8 - ZnO-B ₂ O ₃ -SiO ₂ type glass powder (parts by weight) - 6 - - - - - - 6 Organic vehicle (parts by weight) 35 35 30 35 35 35 35 35 35 _O2 concentration in firing atmosphere (ppm) (whole area) 5 5 5 5 5 5 5 5 5 Peak temperature (℃) 820 780 820 820 820 820 820 820 780 Electrostatic capacitance (μF) 1.03 1.03 1.02 1.04 1.02 1.01 0.95 0.92 0.95 Whether there is foaming none none none none none none have none have Platability ○ ○ ○ ○ ○ ○ × ○ ○ Tensile strength (kg) 3.6 4.6 3.4 3.8 3.8 3.6 2.8 3.7 3.3 Thermal cracks (out of 30 samples) 0 0 0 0 0 0 6 0 twenty three Amount of solder spatter (out of 30 samples) 0 0 0 0 0 10 0 1 4

*Comparative example

From the results shown in Table 1, it is clear that samples 1 to 4 using a mixture of spherical copper powder and flaky copper powder with a glassy thin film were compared with samples 6 and 7 using only one of these powders, or Compared with Samples 8 and 9 using spherical copper powder and flake-shaped copper powder not coated with a glassy thin film, they are excellent in all characteristics. In particular, samples 8 and 9 using spherical copper powder not coated with a glassy thin film exhibited low electrostatic capacitance; this seems to be due to oxidation of the copper powder.

Preparation of samples 10 to 15

The components in Table 2 were mixed separately, except that an amount of octadecylamine ("FARMIN 80" mentioned above) with an average particle size of 7 μm and an average thickness of 0.2 μm or a stearic acid surface treatment as shown in Table 2 was used A conductive paste was prepared using the same materials as in Samples 1 to 9 except for the flaky copper powder. Also, sample 10 is a slurry having the same composition as sample 4, sample 12 is a slurry having the same composition as sample 8, and sample 15 is a slurry having the same composition as sample 5.

Experiment 2

Using conductive pastes with sample numbers 10 to 15, divide the firing atmosphere into a zone where the temperature rises to 600°C (binder removal zone) and a zone where the temperature is higher than 600°C (firing zone), and set the corresponding The atmosphere was a nitrogen atmosphere containing the amount of oxygen shown in Table 2. Also, the peak temperature is variable. Except for this, capacitors were produced in the same manner as in Experiment 1. The same experiment was carried out, and the results obtained are shown in Table 2.

Table 2

Sample No. 10 11 12* 13 14 15 Spherical copper powder coated with glassy thin film (parts by weight) 30 30 - 30 30 10 Spherical copper powder without glassy state film (parts by weight) - - 30 - - - Flake copper powder (parts by weight) 70 70 70 70 70 90 Octadecylamine (weight%) 0.3 - 0.3 0.5 - 0.3 Stearic acid (weight%) - 0.3 - - 0.5 - BaO-ZnO-B ₂ O ₃ type glass powder (parts by weight) 8 8 8 - - 8 ZnO-B ₂ O ₃ -SiO ₂ type glass powder (parts by weight) - - - 10 10 - Organic vehicle (parts by weight) 35 35 35 35 35 35 Binder removal zone _O2 concentration (ppm) 5 20 5 20 5 20 20 50 20 20 50 20 1 1 O ₂ concentration in roasting zone (ppm) 5 20 5 20 5 20 5 5 20 5 5 20 1 1 Peak temperature (℃) 820 820 820 820 820 820 780 780 780 780 780 780 800 840 Electrostatic capacitance (μF) 1.04 1.04 1.00 1.02 0.92 0.76 1.02 1.01 1.01 1.00 1.02 1.00 1.03 1.03 Whether there is foaming none none have none none none none none none have none none none none Platability ○ ○ ○ ○ ○ × ○ ○ ○ ○ ○ △ ○ ○ Tensile Strength(kg) 3.8 3.9 3.4 3.6 3.7 2.2 4.9 4.8 4.9 4.4 4.8 4.6 3.8 3.6 Hot crack (out of 30 samples) 0 0 18 0 0 0 0 0 0 15 0 1 0 0 Solder spatter (out of 30 samples) 0 0 1 0 1 10 0 0 0 0 0 0 0 0

*Comparative example

In samples 10, 11, 13, 14, and 15, spherical copper powder coated with a glassy thin film was mixed with flake-shaped copper powder. It can be seen that samples 10, 13, and 15 treated with octadecylamine to form Excellent terminal electrode, showing little dependence on firing atmosphere, and in the case of binder removal process and high-temperature firing process in an atmosphere containing several tens of ppm of oxygen and only a few ppm or less In the case of carrying out these processes in an atmosphere with a relatively low oxygen content, each has a wide process window. On the other hand, for samples 11 and 14 which treated flake copper powder with stearic acid, the terminal electrode performance can be improved to some extent by adjusting the oxygen concentration in the firing atmosphere in the binder removal zone. Sample 12 is a comparative example using spherical copper powder and flaky copper powder not coated with a glassy thin film; in this case, satisfactory results were not obtained.

Preparation of samples 16 to 18

The components shown in Table 3 were mixed separately, and then in addition to using a certain amount of octadecylamine shown in Table 3 (the above-mentioned "FARMIN 80") with an average particle size of 5 μm and an average thickness of 0.2 μm, dimethyl hard A conductive paste was prepared with the same material as in Samples 1 to 9 except for flaky copper powder surface-treated with tallow amine ("FARMIN DM8098" manufactured by Kao Corporation) or oleic acid as the flaky copper powder. In addition, Sample 18 is a comparative example outside the scope of the present invention.

Experiment 3

Using the conductive paste of samples 16 to 18, an X7R 4.7μF (rated value) capacitor was used as the multilayer ceramic capacitor body, and in addition to setting the firing atmosphere and peak temperature in a nitrogen atmosphere containing oxygen at the concentration shown in Table 3, the Capacitors were produced in the same manner as in Experiment 1. The same test as in Experiment 1 was carried out; the results are listed in Table 3.

table 3

Sample No. 16 17 18* Spherical copper powder (parts by weight) covered with glassy thin film 10 50 - Spherical copper powder without glassy state film (parts by weight) - - 10 Flake copper powder (parts by weight) 90 50 90 Octadecylamine (weight%) 0.5 - - Dimethyl stearylamine (weight%) - 0.7 - Oleic acid (weight%) - - 0.5 BaO-ZnO-B ₂ O ₃ type glass powder (parts by weight) 8 6 12 Organic vehicle (parts by weight) 35 35 35 Binder removal zone _O2 concentration (ppm) 20 5 20 O ₂ concentration in roasting zone (ppm) 5 5 5 Peak temperature (℃) 820 820 820 Electrostatic capacitance (μF) 4.8 4.8 4.0 Blistering present or absent none none have Platability ○ ○ × Tensile strength (kg) 3.2 3.4 1.8 Thermal cracks (out of 30 samples) 0 0 2 Solder spatter (out of 30 samples) 0 0 3

*Comparative example

Obviously, in the results of Table 3, samples 16 and 17, which are mixed with spherical copper powder and flake-shaped copper powder coated with a glassy thin film, are superior to spherical copper powder and flakes that are not coated with a glassy thin film in all characteristics. sample 18 of copper powder.

Claims (6)

1. A conductive paste for terminal electrodes of multilayer ceramic electronic parts, comprising (A) a spherical conductive powder mainly containing copper and having a glassy thin film on at least a part of its surface, (B) a flaky conductive powder mainly containing copper Conductive powder, (C) glass powder, and (D) organic vehicle. 2. The conductive paste according to claim 1, further comprising (E) an aliphatic amine. 3. The conductive paste according to claim 2, wherein at least a part of said (E) aliphatic amine is adsorbed on the surface of (B) flaky conductive powder particles. 4. The conductive paste according to claim 2, wherein the amount of the (E) aliphatic amine is 0.05 to 2.0% by weight based on the weight of the (B) flaky conductive powder. 5. The conductive paste according to claim 1, wherein the weight ratio of (A) and (B) is in the range of 5:95 to 95:5. 6. The conductive paste according to claim 2, wherein the weight ratio of (A) and (B) is in the range of 5:95 to 95:5.